Sulfate recognition by a hexaaza cryptand receptor

Abstract

A hexamine macrobicycle with pyrrolyl spacers was evaluated as an anion receptor in its protonated forms. The protonation constants of the receptor, as well as its association constants with Cl⁻, NO₃⁻, AcO⁻, ClO₄⁻, H₂PO₄⁻, and SO₄²⁻ were determined by potentiometry at 298.2 ± 0.1 K in H₂O–MeOH (50 : 50 v/v) and at an ionic strength of 0.10 ± 0.01 M in KTsO. These studies revealed that the H_npyrrⁿ⁺ receptor has a very high effective association constant value for the SO₄²⁻ at pH 4.0 (log K_eff = 6.42), and it is selective for the uptake of this anion in the presence of the other studied anionic substrates. In particular, the receptor showed very high SO₄²⁻/NO₃⁻ selectivity. Using the indicator-displacement approach the receptor is able to signal the presence of sulfate by a change of color. Single crystal X-ray diffraction determination of [(H₆pyrr)(SO₄)(H₂O)₃](SO₄)₂·9.3H₂O revealed the presence of one sulfate anion inside the receptor cavity and showed that the encapsulation of the anion is favored by an array of nine hydrogen bonding interactions, including N–H⋯O, C–H⋯O and water-mediated ones.